Mobile radios or radio transceivers are also known in the art and referred to herein as mobile stations (MSs). The terms ‘radio’, ‘mobile radio transceiver’ and ‘mobile station (MS)’ are intended to include within their meaning mobile and portable telephones and mobile and portable radio communications units and the like. Systems which include MSs and other units for radio communication with them are referred to herein as mobile radio communications systems.
Mobile radio communication systems may allow any user of the system to establish communication with one or more other users immediately after setting up (i.e. initiating) a call. Such systems generally operate in one of two modes—Direct Mode of Operation (DMO), where there is a direct radio link between mobile stations, and Trunked Mode of Operation (TMO), where the mobile stations are in radio communication via one or more interconnected base stations which are usually deployed in a cellular fashion, the system being actively managed by a system control station. Any particular mobile station may be designed to operate in DMO or TMO or both (in the case of both by switching between these modes). Examples of current commercial systems which provide operation in such modes are known. Systems and mobile stations which operate according to TETRA (Terrestrial Trunked Radio) standards defined by the European Telecommunications Institute (ETSI) are examples.
Communication between a calling mobile station and one or more called mobile stations is implemented in DMO by allocating to the stations a common channel. In TMO this is done by allocating a common channel in each base station. The term “channel” here has its usual meaning in relation to radio communication, such as a carrier frequency (in frequency division multiple access or FDMA systems) and/or a time slot (in time division multiple access or TDMA systems) or a code (in code division multiple access or CDMA). In TMO, there always is one channel, called a “control channel”, over which call initiation signals and identification codes, inter alia, are sent. In DMO, the common channel allocated, is normally used as the control channel, when no call is active.
In many mobile radio communication systems, of both the DMO and TMO types, it is possible to set up a group call between mobile stations of multiple users in a group. The group call is usually implemented by allowing the users to share a channel; sometimes it is also implemented over duplex channels, e.g. in DMO through the use of repeater stations. The system and the over-the-air protocol then support a mechanism to synchronise the channel access between the group members. A group call is arranged by generating a group call set-up request signal in the mobile station of a user wishing to initiate the call and transmitting the signal to the mobile stations of the other group members. This may be actioned in DMO, for example, by the initiator pushing a certain button or key on his/her mobile station, to operate a programmed initiation function in a so-called ‘push-to-talk’ (PTT) or despatch procedure. An appropriate set-up request signal is sent from the mobile station of the initiator over the appropriate control channel. All contactable mobile stations that belong to the called group can thereby pick up the call request and respond, e.g. by automatically joining the call. If a particular mobile station in the group is busy at the time, it may be operable to join the call automatically as soon as it becomes free, by periodic broadcast of a “Late entry” signal.
Normally, within any mobile communications system, one or more groups of mobile stations may be predefined whereby each mobile station belonging to a given group is allowed to have access and join a group call between members of that group. This can be done for example by pre-programming the mobile stations to recognise the group identity, usually by use of a group identity (ID) code or number. Pre-programming a mobile station involves storing therein the ID code of the particular group(s) to which it belongs.
When a group call set-up request signal is initiated over a channel assigned to the group, a corresponding group ID code is transmitted as part of the signal. The group code is entered by the user that is the call initiator. All mobile stations receiving the transmission that have stored this ID code (and thus belong to that group) can detect a match with the stored code and can respond by automatically joining the call or by indicating to the user that a request has been received to join the call.
In current mobile communication systems, access to a group call is generally limited to predefined groups of users identified by their group ID code, as described above. The present inventors have recognised however that such an arrangement can have limitations which in some circumstances can be serious, for the reasons explained later.